Typical commercially available plastic integrated circuits have not been ordinarily used in spacecraft applications because of perceived and real reliability problems. The past practice has been to use only ceramic devices that have been either screened for radiation tolerance or designed to meet high radiation levels. Based on recent improvements in plastic package construction and reliability, the desire to use the latest in commercially available devices, and cost consideration, has resulted in the widespread use of plastic packaged devices. One major concern has been the reliability and the survivability of the devices being considered when exposed to the hazards in the spacecraft environment such as total dose of electrons, protons, and cosmic rays. Typical silicon integrated circuit plastic packaged devices will fail to operate properly when exposed to total doses of 2 to 15 Krads. With the desire to have communication satellites work in orbit for periods of 8 to 15 years, this would rule out the use of almost all commercially available plastic packaged silicon integrated circuit devices.
A typical method of meeting the radiation requirements for space platform environments, known as spot shielding, involves direct application of the radiation shielding materials to the top and bottom of a ceramic integrated circuit device. Aside from the problem that this increases the thickness of the device and increases the weight of the device, it also has the disadvantage that the shield is a significant distance from the integrated circuit silicon die, permitting significant exposure to side angle radiation. Often a bottom spot shield cannot be used due to the offset in height which cannot be accommodated by a fixed device lead length causing problems when used in a printed circuit board "through-hole" package style. Also the shield has no mechanical support except the adhesive used to attach the shield to the surface of the plastic package.
Therefore, it would be highly desirable to be able to effectively shield a commercially available plastic packaged integrated circuit without substantially increasing the size and weight of the integrated circuit package.
Recognizing this problem, Sloan et al. in U.S. Pat. No. 4,468,411 disclosed a method to protect an integrated circuit die which has been prepared for packaging by depositing a liquid polyamide precursor compound on the active surface of the die. U.S. Pat. No. 4,468,411 is incorporated herein by reference. After the initial deposit is cured, additional deposits of the same material could be layered on top of it, thereby increasing the thickness of the protective material. In this way, a sufficiently thick polyamide coating provides alpha particle protection to the die, prior to the plastic encapsulation step.
This technique has several disadvantages. First, the alpha particle protection material must be deposited during the manufacturing process of the integrated circuit. Only after the silicon die has been layered with polyamide is the integrated circuit manufacturing process completed by plastic packaging. More importantly, space for accommodating the polyamide layers is very limited and it is therefore not possible to provide the thickness of protective material required to protect the silicon die from radiation encountered in space platforms.
Therefore, it would be highly desirable to be able to provide effective radiation shielding for commercially available plastic package integrated circuits to be used in various space application environments including low earth orbit, geostationary, or deep space probe.
U.S. Pat. No. 4,833,334 attempts to solve the problem of electronic circuit reliability in the aeronautical and space application environments by providing a protective box within which to house sensitive electronic components. U.S. Pat. No. 4,833,334 is incorporated herein by reference. This protective box is partially composed of a high atomic weight material to shield effectively against X-rays.
Although the protective box is effective in shielding its contents from harmful X-rays, this approach has the serious disadvantage of adding substantial bulk and weight to electronic circuit assemblies protected in this manner. Moreover, it would be expensive to provide this type of protection to individual integrated circuits as manufacturing custom boxes for each circuit configuration would undoubtedly be costly to manufacture. Therefore, the resulting protective boxes cause a significant weight gain, which is undesirable given the costs of placing devices into orbit.
Therefore, it would be highly desirable to have a new and improved method for enabling effective radiation shielding of commercially available package integrated circuits including plastic package integrated circuits, without significantly increasing the bulk or weight of the integrated circuits, in a cost effective and efficient manner.
Another significant problem impacting the protection of integrated circuits intended to be used in outer space is the variation in radiation environments found in different space orbits or trajectories. This significant variation in the amounts and energy levels of electrons, protons, solar particles and cosmic radiation make it very difficult to design a single radiation shielded package that would be universally adaptable to different outer space applications.
Therefore, it would be highly desirable to have a method of formulating radiation shield materials customized to provide radiation protection for integrated circuits, given the particular space application environment in which the integrated circuit is expected to function.